The first detector in a television signal receiver converts radio-frequency (RF) signals in a selected one of the television broadcast channels, which channels occupy various 6-MHz-wide portions of the electromagnetic wave frequency spectrum, to intermediate-frequency (IF) signals in one particular 6-MHz-wide portion of that spectrum. This conversion is typically carried out by superheterodyning the RF signals, which is to say mixing the RF signals with local oscillations from an oscillator oscillating at a frequency substantially higher than the frequencies in the television channel of highest frequency. The first detector is used to convert a selected RF signal to IF signal in order that up to 60 dB or more amplification can be done in that particular 6-MHz-wide portion of that spectrum using intermediate-frequency amplifiers with fixed, rather than variable, tuning. Amplification of the received signals is necessary to raise them to power levels required for further signal detection operations, such as video detection and sound detection in the case of analog TV signals, and such as symbol decoding in the case of digital TV signals. The first detector usually includes variable tuning elements in the form of pre-selection filter circuitry for the RF signals to select among the various 6-MHz-wide television channels and in the further form of elements for determining the frequency of the local oscillations used for super-heterodyning the RF signals. In TV receivers of more recent design the local oscillator signals are often generated using a frequency synthesizer in which the local oscillator signals are generated with frequency regulated in adjustable ratio with the fixed frequency of a standard oscillator.
Television signal receivers for receiving digital television (DTV) signals that have been described in the prior art use plural-conversion radio receivers wherein DTV signal in a selected one of the ultra-high-frequency (UHF) channels is first up-converted in frequency to first intermediate-frequency signal in a first intermediate-frequency band centered at 920 MHz for amplification in a first intermediate-frequency amplifier. The resulting amplified first intermediate-frequency signal is then down-converted in frequency by mixing it with 876 MHz local oscillations, resulting in a second intermediate-frequency signal. This second intermediate-frequency signal, in a second intermediate-frequency band centered at 44 MHz, is then amplified in a second intermediate-frequency amplifier. The response of the second intermediate-frequency amplifier is then synchrodyned to baseband in DTV signal receivers developed by the Grand Alliance.
Radio receivers for receiving DTV signals, in which receivers the final intermediate-frequency signal is somewhere in the 1-8 MHz frequency range, are described by C. B. Patel and the inventor in U.S. Pat. No. 5,479,449 issued Dec. 26, 1995, entitled "DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVER", and included herein by reference. The radio receivers specifically described in U.S. Pat. No. 5,479,449 are of triple-conversion type using a 920 MHz analog IF amplifier for first detector response, the first detector being an up-converter, and using a 44 MHz analog IF amplifier for second detector response, the second detector being a down-converter. A third detector is a further down-converter, generating a 1-8 MHz final IF signal as third detector response. This final IF signal is not amplified, but is digitized by an analog-to-digital converter for use in digital circuitry for synchrodyning to baseband. The resulting digital baseband signal is equalized and then data-sliced in a symbol decoder. The first intermediate-frequency amplifier in one of the DTV signal receivers described in U.S. Pat. No. 5,479,449 uses a surface-acoustic wave (SAW) filter for establishing the bandwidth of the 920 MHz IF amplifier.
The DTV signal receiver developed by the Grand Alliance is of double-conversion type using a 920 MHz analog IF amplifier for first detector response, the first detector being an up-converter, and using a 44 MHz analog IF amplifier for second detector response, the second detector being a down-converter. The amplified response of the 44 MHz analog IF amplifier is the final IF signal, which is synchrodyned to baseband in the analog regime. The resulting analog baseband response is then digitized by an analog-to-digital converter prior to being equalized and then data-sliced in a symbol decoder. The first intermediate-frequency amplifier in The DTV signal receiver developed by the Grand Alliance uses ceramic resonators for establishing the bandwidth of the 920 MHz IF amplifiers.
For a period of years while DTV broadcasting is becoming established, it is planned that the broadcasting of analog TV signals will continue in the United States in accordance with the NTSC standard using the same UHF channels as DTV signals as well as other channels in the VHF and UHF bands. While analog and digital TV signals occupy the same television channels, the requirements of radio receivers for the two types of TV signal are not particularly compatible. Accordingly, there are good reasons for using separate radio receivers for analog TV signals and for digital TV signals in a system designed to receive both types of TV signal.
A reason for using separate radio receivers for analog TV signals and for digital TV signals that will be quite apparent to an electronics design engineer reviewing the systems standards for the two types of TV signals concerns the different radio receiver passbands for each type of TV signal. In an analog TV signal the video carrier is located at a frequency 1.25 MHz above the lower limit frequency of the TV channel, and the vestigial sideband exhibits no gain reduction vis-a-vis the full sideband until modulating frequencies exceed 750 kHz. Accordingly, the radio receiver for an analog TV signal customarily exhibits a linear roll-off of the overall intermediate-frequency response supplied to the video detector, which roll-off is down 6 dB at the video carrier frequency and provides for an overall flat baseband video response up to 4.2 MHz or so. In a DTV signal, the data is located at a frequency only 310 kHz above the lower limit frequency of the TV channel, and roll-off down 6 dB at the data carrier frequency is provided at the transmitter, rather than at the receiver. The overall intermediate-frequency response is essentially flat over a frequency band 6 MHz-wide between 1-dB-down limit frequencies in Grand Alliance receiver designs published by Zenith Radio Corporation.
A radio receiver for an analog TV signal customarily uses a trap filter for removing frequency-modulated (FM) sound carrier from the IF signal supplied the video detector. This is necessary to suppress a 920 kHz beat between the FM sound carrier and the amplitude-modulated chrominance subcarrier, which beat causes unwanted variation in the luminance component of the composite video signal recovered by the video detector. This luminance variation is obtrusively apparent when viewing images reproduced on a television viewscreen. Sound trap filters have not been used in prior-art DTV receiver designs, though co-channel interfering NTSC signals are a known problem during HDTV reception. The avoidance of trap filtering in the IF amplifiers of a DTV signal receiver makes it easier to maintain phase linearity throughout the IF passband.
A more subtle reason for using separate radio receivers for analog TV signals and for digital TV signals, of which one of ordinary skill in the art of design of just one of these types of radio receiver is probably unaware, is the difference in preferred designs of automatic gain control (AGC) for the radio receiver portions of analog TV signal receivers and of DTV signal receivers.
The power in an analog TV signal must be quite high in order that accompanying Johnson or galactic noise is low enough in amplitude as not to cause "snow" (luminance noise) in a black-and-white TV picture or "colored snow" (luminance plus chrominance noise) in a color TV picture. The effective radiated power from an analog TV transmitter is typically tens of kilowatts. The IF amplifier chain in an analog TV signal receiver typically provides maximum gain of 60 to 90 dB, which can be reduced responsive to automatic gain control (AGC). Gain reduction of as much as 66 dB is required to handle the gamut of usable signal strengths. When receiving analog TV signals, this gain reduction is preferably obtained using forward AGC in at least the earlier IF amplifier stages. This avoids the problem of internally generated noise in the IF amplifier stages rising vis-a-vis Johnson noise to adversely affect overall noise figure for the radio receiver, which problem is encountered when using reverse AGC. The great concern with loss in noise figure when receiving analog TV signals arises because the human eye is quite sensitive to the presence of random noise accompanying the composite video signal from the video detector. The amplitude of the luminance signal component of the composite video signal directly controls the intensity of light emanating from or reflected from the television display device, and the amplitudes of the chrominance signal component of the composite video signal directly affect the hue and color saturation of that light.
In a DTV receiver the radio receiver portion thereof supplies plural-level symbol codes as baseband output signal, and the light emanating from or reflected from the television display device is not directly controlled by the amplitude of such baseband output signal. Small amounts of random noise are strongly rejected by quantizing effects in the data-slicing and trellis decoding associated with symbol decoding. Consequently, the overall noise figure for the radio receiver becomes of concern chiefly when distinguishing between the various levels of the symbol codes becomes a problem. In order best to facilitate distinguishing between the various levels of the symbol codes, linearity of the baseband output signal detected by the radio receiver becomes an important concern, and there is less concern for the overall noise figure for the radio receiver unless long-distance reception of DTV signals is sought for transmissions with power levels in the few hundreds of watts.
The AGC of IF amplifier stages in a DTV signal receiver must be such as to avoid non-linearity. Forward AGC tends to introduce non-linearity into the modulation of the IF signal. The resulting distortion is generally tolerable in analog TV signal reception, since larger amplitude modulation properly occurs primarily during synchronizing pulses, and since luminance signal varies in inverse logarithmic relation to scene brightness. Reverse AGC that does not introduce non-linearities into the modulation of the IF signal can be designed for a DTV signal receiver. This can be done using variable-resistance emitter degeneration in a common-emitter transistor amplifier, for example. Or, by way of further example, the collector current of a common-emitter transistor amplifier can be split using common-base transistor amplifiers connected at their emitter electrodes to form a variable-transconductance multiplier. The loss in noise figure with reduction of gain in such reverse AGC arrangements presents little problem as long as overall noise internally generated within the IF amplifier chain of the DTV receiver is smaller than the smallest transitions between digital modulation levels in the final IF amplifier output signal.
The inventor points out that the cost of a first detector is substantial enough that it is undesirable to use separate first detectors for analog TV signals and for digital TV signals in radio receivers designed to receive both types of signal, whether those radio receivers are included in a TV set complete with viewscreen or in a digital recording apparatus, such as one using magnetic tape as a recording medium. The use of a single first detector for both analog rv signals and digital TV signals is also desirable in that it allows more compact radio receiver design and at the same time avoids any problems of unwanted radiation from the output of one of separate respective first detectors for analog TV signals and for digital TV signals to the other first detector.